Lower oceanic crust formed by in situ magma crystallization

Oceanic ridges form a chain of volcanoes in the middle of the oceans, stretching over 60,000 km around the Earth. As the oceanic plates move apart, the mantle rises, causing it to melt at a depth of around 70 km, and magma progresses towards the surface; some of this magma erupts onto the ocean floor while a large portion remains in the crust, probably in the form of magma lenses, and cools and then crystallizes to form the lower crust, scientists propose in a study published in Nature Geoscience, June 13th, 2022. The Moho (Mohorovičić discontinuity) is the boundary that separates this newly formed crust from the underlying mantle.

For this study, a team of researchers from the Institut de Physique du Globe de Paris (IPGP, Université Paris Cité, CNRS) and GEOMAR Helmholtz Centre for Ocean Research in Germany conducted an oceanographic campaign in the equatorial Atlantic Ocean in 2017 aboard the German oceanographic vessel Maria S. Merian. They deployed ocean bottom seismometers (OBS) on the seafloor to record the source of sound energy emitted by the ship and their propagation in the crust. The recorded signals were then analyzed on IPGP’s S-CAPAD supercomputing platform, using the full seismic waveform inversion method developed at the institute. This analysis revealed the presence of 400-500 m thick layers of different rock types starting 2 km below the seafloor. These results indicate that seawater penetrates 2-3 km below the seafloor, cooling the magma and circulating minerals and nutrients to the seafloor at hydrothermal sites (black smokers) feeding millions of living organisms in the ocean depths. The cooled magma then crystallizes on the spot, forming layers of rock of different compositions. These layers extend between 5 and 15 km along the profile studied, covering the crust that formed between 300 and 800 thousand years ago, suggesting that crustal accretion is a stable process.